Austenitic stainless steels form an important group of alloys. Austenitic stainless steels are widely used in many different applications because they have excellent corrosion resistance, ductility and good strength. The annealed austenitic stainless steels are relatively soft. Although there are various ways of strengthening austenitic stainless steels, such strengthening operations often lead to an unwanted reduction of the ductility.
Lately, the introduction of nano twins in metal materials has proven to be an effective way to obtain materials with high strength and high ductility. All materials are however not susceptible to such processing. Further, there is no general operation, by means of which nano twins may be induced into a material. Different methods have been shown to have effects on the inducement of nano twins in different materials. A twin may be defined as two separate crystals that share some of the same crystal lattice. For a nano twin the distance between the separate crystals is less than 1 000 nm.
In US 2006/0014039 a method of inducing nano twins in a metallic foil of stainless steel is disclosed. Stainless steel is sputter deposited to a substrate. The nano twinning is achieved by applying a negative bias to the substrate, which results in a bombardment of Argon ions from the surrounding protective atmosphere. This bombardment alters the intrinsic, growth residual stress of the coating such that controlled layers of twins are formed. The method described is thus only applicable on the production of coatings or foils, and not on integral pieces of metal.
EP 1 567 691 discloses a method of inducing nano twins in a cupper material by means of an electro deposition method. The method is however restricted to function on copper materials.
Another possible way of introducing nano twins into metal materials is to plastically deform the material. One example is given in the scientific article “316L austenite stainless steels strengthened by means of nano-scale twins”, (Journal of Materials Science and Technology, 26, 4, 289-292, by Liu, G. Z., Tao, N. R., & Lu, K). In this article a method of inducing nano scale twinning by plastic deformation at high strain rates is described. The strength of the material is thus increased. On the other hand the plasticity (ductility) of the nano twinned material is very limited, with an elongation-to-failure of about 6%. To improve the plasticity, the plastic deformation needs to be followed by a thermal annealing in order to partially re-crystallize the deformed structure.
Even though there are successful examples of increasing the strength of austenitic stainless steels there is no general method of inducing nano twins that functions over the whole composition span of austenitic stainless steels. Further, no twin induced plasticity (TWIP) in austenitic steels has been reported. TWIP signifies that the formation of twins has occurred during plastic deformation and that as a result thereof an increase of both the strength and the ductility or elongation has been achieved.